1. Field of the Invention
This invention relates generally to transmit/receive (T/R) circuit modules utilized, for example, in phased array radar systems and, more particularly, to a dual channel T/R module where two discrete T/R RF signal channels are implemented side-by-side in a common package.
2. Description of Related Art
Phased array radars utilizing electronically scanned antenna arrays, also referred to as active apertures, require many individually controllable T/R modules which are arranged in an array. The T/R modules are connected to frontally located radiator elements which collectively generate a transmitted radar beam. The beam is normally energized, shaped and directed in azimuth and elevation under electronic control of the signals applied to the individual radiators.
A phased array radar system generates successive transmit pulses which are distributed through a transmit manifold and microwave circuitry to the various antenna radiators. Between transmit pulses, the radar system receives and processes successive return signals from the antenna radiators. The return signals are processed through microwave circuitry in the T/R module, collected through a receive manifold, and then processed in the system for target identification.
Such a radar system also employs a programmed digital processor to control amplification, attenuation, and phase shifting of transmit and receive signals, thereby determining the amplitude, direction, and shape of the aggregate RF energy beam transmitted by or received by the aperture. Different phase shifts cause different transmit or receive circuit delays in delivery of individual RF radiator signals to control the pattern of RF energy wavefronts associated with the different radiators and which are combined to define the direction and shape of a transmitted or received antenna beam.
Each T/R module according to the known prior art typically includes a housing structure or package including microwave signal processing means for processing transmitted and received radar signals, control signal processing means interconnected with microwave signal processing components for coupling control signals thereto; and power conditioning means comprising a number of power conditioning components selectively interconnected with the microwave signal processing components and the control signal processing components for providing electrical power thereto. Because such apparatus operates at relatively high power levels, there is also normally provided means for dissipating the heat generated by the various components, particularly the microwave power amplifiers and the power conditioning components associated therewith.
Accordingly, there is an ongoing development in this field of microwave technology to produce T/R modules that are smaller, lighter in weight and lower in cost, while at the same time providing an improvement in operating performance and reliability as well as enhancing ease of installation in an antenna assembly.
One known T/R module developed by the assignee of this invention is shown and described in U.S. Pat. No. 4,967,201, entitled, "Multi-Layer Single Substrate Microwave Transmit/Receive Module", granted to Edward L. Rich, III, on Oct. 30, 1990, one of the inventors named in this application. The module disclosed therein is referred to as a "sugar cube" T/R module and includes a single multi-layer substrate having at least two opposed mounting surfaces. The substrate includes a plurality of integrated dielectric layers, electrical conductors and thermal conductors selectively interconnected between the layers of the substrate. Microwave signal processing means is mounted on at least one of the mounting surfaces of the substrate for processing microwave radar signals. Control signal processing means is also mounted on at least one of the mounting surfaces of the substrate for providing control signals for the microwave signal processing means. Power conditioning means is additionally mounted on at least one of the mounting surfaces of the substrate for providing power to power the microwave signal processing means and control signal processing means. A heat sink interface is coupled to a set of thermal conductors or vias passing vertically through the substrate layers and which are positioned in thermal proximity to selected portions of the microwave signal processing means, the power conditioning means, and the control signal processing means for conducting thermal energy away from the heat generating elements mounted on the substrate to a heat sink.
The "sugar cube" module comprises a relatively early T/R module design in which basic transmit and receive functions, as then conceived, are embodied in a single modular T/R unit with the operating structure supporting such functions integrated together on a main substrate. While presumably operating as intended, certain inherent deficiencies have been found to exist. For example, while the "sugar cube" module exhibits a compact appearance, it embodies only a single T/R channel and is limited by its design to relatively low RF power output operation and is structurally limited to a single RF connection to an RF manifold. Also, while this type of module has a back-end plug-in capability for certain electrical connections, it has no easy plug-in capability for antenna connections. Instead, each module has an antenna radiator built into its front end, thereby creating installation problems in aligning misaligned radiators among installed T/R modules. This is due to the fact that transmitted and received beams require aligned antenna radiators to enable beam control in accordance with system commands.
Moreover, the module-integrated radiator of the "sugar cube" module limits bandwidth during transmission and reception and, because of its simple unpolarized patch structure, restricts radiator operation to a fixed polarization. The "sugar cube" T/R module is thus characterized with polarization inflexibility, whereas good system design requires polarization flexibility to permit variable settings of radiation properties including bandwidth and polarization. For example, if a received signal carries a high noise level in a particular polarization, it is desirable to have the flexibility to control the polarization to an angle where the noise is reduced. In this manner, the signal-to-noise ratio is enhanced and weaker signals can be detected with substantially reduced noise interference.
Further, in an antenna assembly employing "sugar cube" T/R modules, the pin within the single coaxial RF connector between each sugar cube module and the system manifold is susceptible to excessive axial movement in response to antenna mechanical vibrations. Such pin movements can change RF path lengths thereby causing increased noise level and erroneous phase changes which produce beam dispersion and thereby affect intended beam control.
Among other problems encountered with the "sugar cube" T/R module is the removal of heat generated by the active components therein. Thermal conductors, coursing vertically through the layered structure of the module to a heat transfer interface provides only limited heat transfer for removal of heat from the active circuit components. As a result, poor thermal performance contributes to a relatively low RF-power-output capability.
More recently, an improved T/R module has been developed by the assignee of this invention and is disclosed in U.S. Pat. No. 5,745,076, entitled "Transmit/Receive Module For Planar Active Apertures", issued to Thomas R. Turlington et al on Apr. 28, 1998. The T/R module disclosed therein and referred to by the assignee as a "StackPak" comprises a module configuration which plugs into the backside of an active aperture and includes discrete RF, DC power and data distribution manifolds which are planar in configuration and are stacked together one on top of the other between a cold plate and an antenna assembly, with the antenna elements and circulators being assembled in a single physical unit which forms the front layer of the aperture.
The T/R module itself comprises a multi-chip microwave package comprised of multiple layers of high temperature cofired ceramic (HTCC) including ground planes, stripline, data and DC interconnects, thermal vias and RF inputs/outputs running through the RF assembly for a plurality of monolithic microwave integrated circuit chips (MMICs) which are located in cavities formed in the RF assembly layer. The module's architecture includes a single transmit/receive RF signal channel that shares its control functions of gain trim, phase shift and intermediate power amplification in both the transmit and receive modes of operation.
When "StackPak" T/R modules are installed in place, they are disposed against the cold plate for removal of internally generated heat. Each T/R module, moreover, has connector pins extending forwardly from a front module side to make all power, control and RF connections required for the module when it is installed by plugging into the back of the antenna assembly. The forwardly extending pins pass through respective sleeves which, in turn, extend through the stacked layers, thereby enabling the necessary connections to be made between the pins and the antenna radiators, the RF manifolds, and the control and power systems in the various layers.
The "StackPak" scheme thus resembles "Swiss cheese" in the sense that the sleeves pass through assembly openings to provide for the necessary DC power, DC digital control, and RF signal connections for the T/R modules.
In dissipating heat to a heat exchanger, a "StackPak" T/R module can use only a portion of its front surface for the dissipating heat transfer. Gallium arsenide integrated circuits are normally used for RF power amplification in T/R modules, and the temperature and reliability specifications for these devices require increasing heat dissipation for increasing power rating. Thus, "StackPak" T/R modules exhibit relatively poor heat dissipation, and consequently restrict RF power generation, largely because the frontal "real estate" of the T/R module must share heat transfer and electrical connection functions thereby operating with a highly restricted surface area for heat removal.
While the substrate-based structure of the "StackPak" employs cavities in an RF assembly layer for placement of various RF semiconductor devices to support RF circuitry in a single RF channel, there is no provision for semiconductor device layout or RF circuit routing and RF shielding and isolation between or among two or more discrete T/R channels.
The "StackPak" T/R module is also limited by the fact that it employs RF input/output coaxial connectors on three different edges of the module thereby adversely affecting module installation facility, RF circuit length, RF power loss, and RF channel isolation.
The "StackPak" T/R module is furthermore hindered by limited capacity for interfacing DC power from an external power supply to the T/R module. Thus, in an active aperture, a low voltage bus normally supplies power to T/R modules from an external power supply, i.e., a DC converter which converts a main source voltage (such as 240V) to a low DC voltage (such as 10V or 11V) for module use. The weight of the low voltage DC (LVDC) bus increases in proportion to the square of the length of the bus path and in proportion to the square of the current carried by the LVDC. Increased RF output power requires increased transmit current pulses, which place increased peaking current demands on the input power supply circuitry, i.e., increased bus path cross-section and weight if increases in bus power losses and heat generation are to be avoided. These principles also apply to any input LVDC path length connected to the external LVDC bus path and extending within the T/R module to power distribution points. However, the internal LVDC bus path length would normally be relatively short and have less significance to bus power loss and heat generation than the external LVDC bus would have. In accordance with good design practice, the LVDC bus structure desirably keeps losses at or below a specified percentage of RF power output as a control on efficiency in producing output RF output power. Thus, RF power output increases require significantly increased LVDC bus size and weight.
Thus, the design of the "StackPak" T/R module substantially affects the RF power output, since excessive bus size and weight is required to reach desirable levels of RF output power. Other factors including poor heat dissipation also limits RF output power in the "StackPak" design. Although the module can achieve some cost improvement through chip-to-chip wire bonding, but it still carries cost disadvantages resulting from factors including the use of multiple housing/interconnect/seal pieces.
Notwithstanding the advances made in the art by the above-mentioned T/R modules, there is nevertheless an ongoing need for improvements, which result in reduced weight, cost and size, while at the same time maintaining required performance parameters.
In achieving further development of T/R modules, it is desirable that the following objectives be met: (1) Maximum RF output power; (2) Minimum shielded RF circuitry routing within module; (3) Minimum received noise figure; (4) Maximum isolation between RF channels to facilitate proper beam steering and shaping; (5) Phase adjustability for facilitated beam steering; (6) Minimum heat generation; (7) Minimum thermal resistance allowing maximum heat dissipation; (8) Minimum semiconductor junction temperature rise; (9) Minimum inflow power current; (10) Minimum logic circuit routing; and, (11) Relative ease of installation in an antenna assembly.
In summary, the prior art in its previous and current states has generally been deficient in meeting the above mentioned objectives individually and collectively. The subject invention, therefore, is directed to an improved T/R module which meets these objectives while providing reduced cost, greater reliability, and greater maintainability.